Some current optical devices for holographic data storage, diffractive optical elements (DOEs), holographic optical elements (HOEs), gradient refractive index (GRIN) materials, optical circuits, and optical waveguides use an organic polymer matrix in conjunction with a photopolymerizable material to record light intensity dependent gratings. The inherent advantages of such systems are ease of fabrication, high dynamic range, high sensitivity, good archival life of recorded gratings, and good shelf life. The easiest way to increase the dynamic range of these particular systems is to increase the amount of photopolymerizable material. However, as the photopolymerizable material polymerizes during recording, the photopolymerizable material can shrink, which distorts the optical devices and the holographic recordings thereon. Distortion of the gratings hinders readout of recorded data and changes the wavelength of reflection for display holograms.
Additionally, the sensitivity of grating formation is dependent on the concentration of the photopolymerizable material (monomers and photoinitiators); the sensitivity increases with increases in concentration of photopolymerizable material. Yet again though, shrinkage prevents arbitrarily increasing the concentration of photopolymerizable material. One way to circumvent this concentration limitation is to use a preformed rigid matrix support; for example, a matrix can be made from aerogel, xerogel or from Vycor (a porous silicate glass made by Coming Incorporated). These preformed rigid supports contain channels in which the photopolymerizable material can be inserted. For example, U.S. Pat. No. 6,077,629 to Parker et al. discusses the use of an aerogel material (usually comprised of silica) to form a rigid glass matrix. A photopolymerizable material is infused into the aerogel matrix and then sealed as a single unit. Parker et al. indicate that such a device eliminates problems associated with polymerization induced shrinkage and thus allows for increased dynamic range and sensitivity.
However, one of several disadvantages of using an aerogel, xerogel or Vycor as a supporting matrix is that the matrix is made independently of the device and then the photosensitive material is infused into aerogel or xerogel matrix (as mentioned above). Another disadvantage is that the loading of the photosensitive material into the porous glass structure is difficult (resistance to flow can be high depending on the photosensitive material) and sometimes air can be entrapped in the matrix during loading.
Yet another disadvantage is that without added crosslinker or other mechanism, a recorded spatial light intensity pattern can decrease with time. This is because the formed photopolymer can slowly diffuse through the channels (when the channels are large). In the very least, the use of a porous inorganic glass material involves the additional manufacturing step of infusing the photosensitive material into the porous inorganic matrix as compared to organic matrix systems wherein the organic matrix is formed in situ as discussed in U.S. Pat. No. 6,103,454 to Dhar et al. (The latter organic matrix was not mesophasic).
Aerogels, xerogels, and vycor like materials are all made with a templating agent. The templating agent helps to form the desired mesophase. Typical sol-gel reactions occur during and after mesophase formation, creating at least one inorganic phase. The templating agent is then removed. The article is then typically heated to drive off all templating agents, solvents, and condenstates. As mentionsed above, it is at this stage that the material is then infused with photosensitive components forming an organic-inorganic hybrid mesophasic material.
Independent of holographic data storage technology, the field of organic mesophasic materials has enjoyed much attention in the past two decades. Previously, aspects of this field of research were used to develop liquid crystal displays and a variety of novel composite materials. However, more recent interest has been in the development of materials with special morphologies such as hexagonal, lamellar, and interpenetrating cubic phases. Where as the former phases are more widely known and fairly well characterized, the cubic phase is less known and less characterized. See D. L. Gin, et al., Acc. Chem. Res., 34, 974–980, 2001. The unique material properties afforded by the interpenetrating cubic phase has similar characteristics to that of sol-gels and porous inorganics in that two distinct phases are present. Additionally, depending on the molecular size of the constituents that form the cubic phase, the resultant material can be clear and free of light scatter. See I. W. Hameley, et al. Langmuir, 18, 1051–1055, 2002.
Uses for these organically derived mesophasic systems have been limited compared to their inorganic counterparts by the transient nature of the phases themselves. For instance, the phases of the organically derived mesophasic systems typically exist only in small temperature ranges and with specific concentrations of the constituents. Recent work has been done to lock down the structure of the organic mesophasic systems by free radical polymerization of functional groups located on the constituent molecules, for example, see D. L. Gin, et al., Acc. Chem. Res., 34, 974–980, 2001; E. Tsuchida, et al., Macromolecules, 25, 207–212, 1992; M. Jung, A. L. German, H. R. Fischer, Colloid Polym Sci 279, 105–113,2001; S. Liu and D. F. O'Brien, Macromolecules, 32, 5519–5524, 1999; W. Srisiri, et al., Langmuir, 14, 1921–1926, 1998; and T. M. Sisson, et. al., Macromolecules, 29, 8321–8329, 1996. Using free radical polymerization of functional groups did extend the thermal range and stability of the locked in phase when the polymerization was performed quickly, as is typically the situation in photopolymerization. Other methods of locking the phase have been to polymerize a polymer into the channels of the mesophasic systems to lend structural support to the channel system. See C. L. Lester, et. al., Macromolecules, 34:25, 8587–8589, 2001. This method has shown some promising results for stabilizing the phase and some interesting polymerization kinetic data was also revealed. For instance, polymerization rates are enhanced in some mesophasic materials. See C. L. Lester, et. al., Macromolecules, 34:25, 8587–8589, 2001).
The organic mesophasic materials represent an advantage over previous inorganic mesophasic materials in that the templating agents can often be left in the material or that the material itself naturally forms mesophases. However, previous organic-inorganic sol-gel materials were structurally more stable. A combination of the mechanical stability of the sol-gel materials and the ease of formation of the organic mesophasic materials is desirable.